In a known fuel pump assembly a pumping plunger is driven for reciprocal movement within a bore provided in a pump housing by means of a cam drive arrangement including an engine-driven cam. A cam follower cooperates with the cam, and in turn a foot of the plunger cooperates with the cam follower. Fuel from a low pressure fuel source is delivered to a pumping chamber via an inlet metering valve which controls the rate of flow of fuel into the pumping chamber. As the plunger reciprocates within the bore fuel within the pumping chamber is pressurised. An outlet valve controls the delivery of pressurised fuel to the downstream common rail.
In one type of unit pump arrangement the pump assembly includes a plurality of separate pumps units, each with its own pumping chamber, inlet valve and outlet valve. A cam follower is provided for each of the pump units, each cam follower being cooperable with a respective cam that is carried on a drive shaft common to the other cams.
In other arrangements, such as described in EP 0778413 B, the plungers are arranged as opposed pairs and are driven inwardly by a radially outer cam ring to perform a pumping stroke during which fuel within a pumping chamber, arranged between opposed ones of the plungers, is pressurised. Each plunger has an associated cam follower which cooperates with the cam ring, with an inlet metering valve and an outlet valve being provided for each pair of the plungers. As the cam followers ride over the cam surface the plunger pairs are driven inwardly to reduce the volume of the pumping chamber and, hence, increase fuel pressure within the pumping chamber.
Typically the inlet metering valve takes the form of a variable orifice which controls the flow of fuel into the pumping chamber. The volume of fuel to be pressurised in any given plunger stroke will usually be less than the maximum swept volume of the associated pumping chamber. Particularly in pump arrangements for which a degree of relative movement is permitted between each plunger and its cam follower this can give rise to an impact load between the foot of each plunger and its cam follower when they come into contact during the pumping stroke. The impact load can lead to wear at the interface between the plunger and its cam follower and may give rise to mechanical noise and other undesirable side effects.
It is with a view to addressing or mitigating the aforementioned problem that the present invention provides an improved fuel pump assembly and plunger/cam follower arrangement.